1. Field of the Invention
The present invention relates to a surveying instrument having a phase-difference detection type focus detecting device.
2. Description of the Related Art
Advancements have been made in the development of surveying instruments (e.g., a total station) provided with a sighting telescope (collimating telescope) having an autofocus system, wherein phase-difference detection type autofocus system is widely used in the autofocus operation therefor. With this system, an in-focus state is detected based on the correlation between two images formed by two light bundles which are respectively passed through two different pupil areas upon passing through different portions of an objective lens of the sighting telescope to automatically focus the sighting telescope on a sighting object in accordance with the detected in-focus state.
A typical surveying instrument provided with a sighting telescope having a phase-difference detection type autofocus system adopts a Porro-prism erecting system as an erecting optical system for changing an inverted image formed through the objective lens of the sighting telescope into an erect image. One of the four reflection surfaces of the Porro-prism erecting system is formed to serve as a beam splitting surface which splits the incident light bundle into two light bundles so that one of the two light bundles proceeds toward a phase-difference detection type AF sensor unit (phase-difference detection type focus detecting device) while the other light bundle proceeds toward the eyepiece of the sighting telescope. Although each of the four reflection surfaces can be formed as such a beam splitting surface, no attention has been directed to the matter of which of the four reflection surfaces is the most appropriate to be formed as the beam splitting surface. Conventionally, when one of the four reflection surfaces is formed as the beam splitting surface, the whole effective area of the reflection surface which reflects light bundles traveling toward the eyepiece of the sighting telescope is formed as a semitransparent surface (e.g., a half-silvered surface), while a beam splitting prism is adhered to the whole surface of the semitransparent surface. Due to this structure, the beam splitting prism projects largely from the Porro-prism erecting system, which makes it difficult to design a small sighting telescope. Moreover, the field of view of the sighting telescope becomes dark when a surveyor views a sighting object through the sighting telescope since the semitransparent surface spreads over the whole field of view of the sighting telescope, i.e., the whole field of view of the sighting telescope is viewed through the semitransparent surface.
The present invention has been devised in view of the problems noted above, and accordingly, an object of the present invention is to provide a compact surveying instrument having a phase-difference detection type focus detecting device and a Porro-prism erecting system provided with a beam splitting surface, wherein a bright field of view of the sighting telescope is achieved.
To achieve the objects mentioned above, according to an aspect of the present invention, a surveying instrument is provided, including a sighting telescope having an objective lens and an eyepiece for sighting an object; a Porro-prism erecting system positioned between the objective lens and the eyepiece; a semitransparent surface formed on a reflection surface of the Porro-prism erecting system; a beam splitting prism adhered to the semitransparent surface; and a phase-difference detection type focus detecting device which is positioned to receive light which is passed through the semitransparent surface and the beam splitting prism to detect a focus state from a correlation between a pair of images respectively formed by two light bundles which are respectively passed through two different pupil areas of the objective lens and passed through two separate areas on the semitransparent surface. An area of a surface of the beam splitting prism which is adhered to the semitransparent surface is smaller than an effective area of the reflection surface of the Porro-prism erecting system. The semitransparent surface and the beam splitting prism are positioned so that the two light bundles which are respectively passed through the two different pupil areas of the objective lens are incident on the semitransparent surface at the same incident angle.
In an embodiment, the reflection surface of the Porro-prism erecting system, on which the semitransparent surface is formed, constitutes a first reflection surface of the Porro-prism erecting system. A width of the beam splitting prism is identical to a width of the first reflection surface, and a length of the beam splitting prism, which is adhered to the semitransparent surface, is shorter than a length of the first reflection surface, the semitransparent surface having an elongated shape. The elongated shape of the beam splitting prism is determined so that the two light bundles which are respectively passed through the two different portions of the objective lens are incident on the first reflection surface at the same incident angle.
In an embodiment, a portion of the first reflection surface to which the beam splitting prism is not adhered is covered by a semitransparent material so that variation of the phase angle of light before and after the light is incident on the portion of the first reflection surface becomes approximately equal to the phase angle before and after the light is incident on the semitransparent surface.
In an embodiment, the semitransparent surface is formed on the whole effective area of the first reflection surface.
Preferably, the length of the beam splitting prism, in a direction of the optical axis of the sighting telescope, is less than a thickness of the Porro-prism erecting system.
In an embodiment, the Porro-prism erecting system includes three right angle prisms which include the first reflection surface, a second reflection surface, a third reflection surface and a fourth reflection surface.
In an embodiment, the portion of the first reflection surface is covered by the semitransparent material to be formed as a semitransparent surface.
In an embodiment, a light shield mask is further provided, positioned between an exit surface of the beam splitting prism and phase-difference detection type focus detecting device.
In an embodiment, the sighting telescope includes a focus adjustment lens positioned between the objective lens and the Porro-prism erecting system.
The present disclosure relates to subject matter contained in Japanese Patent Application No.2000-138313 (filed on May 11, 2000) which is expressly incorporated herein by reference in its entirety.